1. Field of the Invention
The present invention relates to a solid-state imaging element having, above a photodiode, an optical waveguide for guiding incident light into the photodiode.
2. Description of the Related Art
It is known in CCD (charge coupled device) and CMOS (complementary metal-oxide semiconductor) solid-state imaging elements that a crystalline defect in a photodiode and an interface state at the interface between a photoreceiving section and insulating layer on top thereof formed in a silicon substrate are attributable to dark current.
That is, as illustrated in the diagrammatic sectional view of FIG. 10A and the potential diagram of FIG. 10B, an interface state denoted by “x” develops at the interface between a silicon layer 51 and insulating layer 52 on top thereof. A photodiode PD is formed in the silicon layer 51. This interface state serves as a source of dark current, causing electrons originating from the interface to flow into the photodiode PD as a dark current.
The so-called HAD (hole accumulation diode) structure is employed to control the generation of dark current caused by an interface state among various causes of dark current (see, Japanese Patent Laid-Open No. 2005-123280 for example).
More specifically, as illustrated in the diagrammatic sectional view of FIG. 11A and the potential diagram of FIG. 11B, a p-type impurity is introduced in the vicinity of the surface of the silicon layer 51 to form a p+ semiconductor region. This p+ semiconductor region serves as a positive charge accumulation region 53 adapted to accumulate positive charge (holes).
Thus, the photodiode PD is separated from the interface if the HAD structure is provided by forming the positive charge accumulation region 53 at the interface. This makes it possible to suppress dark current caused by an interface state.
In order to form the HAD structure, B, BF2 or other ions are injected at an annealing temperature to form a p+ semiconductor region in the vicinity of the interface as the positive charge accumulation region 53.
In related art solid-state imaging elements, on the other hand, the provision of an optical waveguide having a well structure in the wiring layer on the photoreceiving region of the photoelectric conversion section (photodiode) is known to provide improved light collection efficiency onto the photoreceiving region (see, Japanese Patent Laid-Open No. 2003-224249 for example).
This configuration relies on a higher refractive index of the transparent core material in the optical waveguide than the interlayer insulating film (SiO2). This configuration can provide improved light collection efficiency into the photodiode by totally reflecting incident light having an incidence angle greater than the critical angle which is determined by the interface between the transparent core material and interlayer insulating.